The present invention relates to a sheet alignment device which corrects the skew (or oblique movement) of paper during conveyance or a positional displacement of paper in a direction intersecting the direction of conveyance. Further, the present invention relates to an image forming apparatus, such as a copier, equipped with the sheet alignment device.
In an image forming apparatus, in a case where paper is skewed during conveyance or is displaced in a direction intersecting a direction in which a sheet is conveyed (hereinafter simply referred to as a direction of conveyance), an image is formed in a position offset from the paper. Particularly, in a copier having double-sided copying capability, after an image has been formed on a first surface, another image is formed on a second surface by inversion of paper through use of an inverting device. If paper is skewed during conveyance or is displaced in a direction intersecting the direction of conveyance, the images on the first and second surfaces deviate from each other.
For this reason, in order to correct skewing of paper during conveyance or side misregistration, such as positional displacements of paper in a direction intersecting the direction of conveyance, a sheet alignment device is employed. Various types of sheet alignment devices have already been proposed, and representative examples of such devices will be mentioned hereinbelow.
In a known existing sheet alignment device, a pair of stoppers are situated in different positions on both sides of a sheet transport path in a direction intersecting the direction of conveyance in such a way as to advance or retract. A lead skew of the paper is corrected by bringing the leading edge of the paper into contact with the pair of stoppers while the paper is being conveyed. Subsequently, the pair of stoppers are retracted from the sheet transport path (see; for example, the Unexamined Japanese Patent Application Publication No. Sho 63-225052).
In another known existing sheet alignment device, a pair of paper sensors are situated in different positions on both sides of the sheet transport path in the direction intersecting the direction of conveyance. The amount of skew is calculated from a difference between the instant when one end of the leading edge of paper passes by the sensors and the instant when the other end of the leading edge of the paper passes by the sensors. The lead skew is corrected by independent control of the rotational speed of two conveyor rollers, which are spaced apart from each other in the direction intersecting the direction of conveyance, according to the thus-obtained amount of skew (see; e.g., the Unexamined Japanese Patent Application Publication No. Hei 3-53219).
In still another known existing sheet alignment device, a reference wall is disposed on one side of the sheet transport path, and skew rollers are disposed on the sheet transport path. The skew rollers draw paper toward the reference wall during conveyance, to thereby bring the side edge of the paper into contact with the reference wall. As a result, a side skew of the paper is corrected, and side registration of the paper is accomplished simultaneously (see; e.g., the Unexamined Japanese Patent Application Publication No. Sho 57-90344).
In yet another known existing sheet alignment device, conveyor rollers used for the purpose of carrying paper are disposed so as to be movable in the axial direction of the conveyor roller. A paper sensor used for the purpose of detecting the side edge of paper is disposed in the reference position for side registration in the vicinity of the conveyor rollers. This paper sensor detects whether or not the side of the paper being conveyed is in the reference position. The side registration of the paper is accomplished by moving the conveyor rollers in the axial direction on the basis of the result of detection (see; e.g., the Unexamined Japanese Patent Application Publication No. Sho 59-4552).
The existing sheet alignment device described in the Unexamined Japanese Patent Application Publication No. Sho 63-225052 is configured so as to bring the leading edge of paper into contact with one of the stoppers. Accordingly, the lead skew of the paper can be corrected. However, the side registration of the paper cannot be accomplished. Further, a sound is produced when the paper abuts the stopper, and the paper is temporarily stopped, thereby resulting in deterioration of productivity.
Further, there is only a narrow range of correctable skews, and there is a difference in the degree of parallelism between the leading edge and trailing edge of the paper. For this reason, in the case of a copier having double-sided copying capability, the leading edge and trailing edge of the paper are interchanged with each other when the paper is inverted by the inversion device. If the skew of the paper is corrected by bringing the leading edge of the paper into contact with the stopper, images formed on the first and second surfaces deviate from each other.
The existing sheet alignment device described in the Examined Japanese Patent Application Publication No. Hei 3-53219 is structured so as to correct skew with reference to the leading edge of paper, as is the previous existing sheet alignment device. Therefore, it is impossible for the device to accomplish side registration of the paper, so that images formed on the first and second surfaces deviation from each other during double-sided copying operation. Further, there is a need to calculate the amount of skew from a difference between the instant when one end of the leading edge of paper passes by the pair of paper sensors and the instant when the other end of the leading edge of the paper passes by the pair of paper sensors. A velocity profile of the conveyor rollers must be calculated from the amount of skew. Expensive calculation means is required for the purpose of calculating the velocity profile. If the conveyor rollers are abraded, the accuracy of correction of skew is correspondingly deteriorated.
The existing sheet alignment apparatus described in the Unexamined Japanese Patent Application Publication No. Sho 57-90344 is configured so as to accomplish the side registration of paper by bringing the paper into contact with the reference wall by means of the carrying force of the skew rollers. If the carrying force of the skew rollers is too great, thin paper may become buckled. In contrast, if the carrying force is too weak, it becomes impossible to carry thick paper to the reference wall. For these reasons, the range of applicable paper quality is narrow. In addition, the skew rollers are apt to abrasion, and the accuracy of side registration of paper may change according to paper quality.
The existing sheet alignment apparatus described in the Unexamined Japanese Patent Application Publication No. Sho 59-4552 is configured so as to align the side edge of paper with the reference position only by transverse movement of paper in parallel with the direction of conveyance (or in the direction orthogonal to the direction of conveyance) while the paper is being conveyed. Accordingly, it is impossible for the device to correct skewing of paper. The device is able to yield an advantage only when paper is conveyed without being skewed.